Photoreceptors can transmit minute changes in voltage to photosynaptic cells over a wide range of background light intensities. Knowledge of the processes and molecule; involved in normal synaptic transmission from these specialized cells is essential if we wish to know how they might be altered in degeneration and disease. We have developed a prepartion, from an invertebrate, consisting-of huge photoreceptors with giant presynaptic terminals and recognizable postsynaptic cells, and have identified the photoreceptors' neurotransmitter as histamine. The particular advantages of this preparation will be exploited to study cellular processes, particularly those of transmitter dynamics, less easily approached in vertebrate photoreceptors. We will follow the fate of transmitter molecules with LM and EM autoradiography control and record the voltage from pre- and postsynaptic cells with electrophysiological methods and follow changes in intracellular calcium concentration using highspeed optical recording. The following questions will be addressed: (1) How does the reuptake of released transmitter contribute to synaptic function, in particular signal transfer? How is uptake into photoreceptors regulated, especially in a cell that releases transmitter continuously as this one does, and what role is played by uptake into glial cells at this synapse? (2) How does the synthesis of transmitter contribute to synaptic function and how is it regulated? (3) Is transmitter released in a quantal fashion from this synapse or by a carrier, and where is it stored? (4) Can we control the internal milieu of these terminals and in this way attempt to dissect out the steps in the release process?